A receiver operating on the RAKE principle has a plurality of branches, each branch being capable of synchronizing to a different signal component. Consequently, the receiver is able to receive several signals simultaneously. RAKE receivers are used in CDMA receivers.
A CDMA (Code Division Multiple Access) system is a multiple access method which is based on spread spectrum technology and whose application in cellular radio systems has lately been initiated along with the earlier FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access) technologies. The CDMA technology has several advantages over the earlier methods, such as spectral efficiency and simple frequency planning.
In a CDMA method, the narrow-band data signal of the user if multiplied by a spreading code of much wider bandwidth to a relatively wide band. In known experimental systems, the bandwidths used include, for example, 1.25 MHz, 10 MHz and 25 MHz. In the multiplying process, the data signal spreads across the entire band. All users transmit simultaneously by using the same frequency band. A separate spreading code is employed for each connection between a base station and a mobile station, and the signals from the users can be identified from one another in the receivers based on the spreading code of each connection. An attempt is made to select spreading codes so that the spreading codes are mutually orthogonal, i.e., do not correlate with each other.
In a typical radio system, such as a cellular radio system, a subscriber terminal communicates with one base station only. In, e.g., the CDMA system, a subscriber terminal may, however, also communicate with several base stations at the same time. In a prior art soft hand-off, an unbroken connection is maintained with a base station regardless of the hand-off. In such a hand-off, the base station typically changes. Also known in prior art is a softer hand-off in which the base station does not change, but the sector of the base station employed does. The soft and softer hand-offs are referred to as make-before-break type of hand-offs, which means that a new connection is established for a subscriber terminal before the previous base station connection is cut. Neither of these hand-offs change the frequency band used.
Cellular radio systems include cells that have been divided in sectors. A softer hand-off may be implemented, e.g., by placing in each sector a RAKE receiver which receives a signal. The signals received by the RAKE receivers placed in the sectors are routed via a bus to a combiner which carries out diversity combining for the signals. Diversity combining reduces the disadvantages caused by a fading signal. In practice, however, routing of signals to a combiner via a bus is difficult to implement because the implementation requires a very fast bus. In a cellular radio system, a base station receives a signal from the radio path by an antenna. Connecting the signals received by the antenna to a RAKE receiver causes additional problems. Problems occur particularly in case of a high capacity CDMA base station, which signifies a large number of signals to be connected.
Attempts have been made to overcome the aforementioned problems by grouping the RAKE receivers in a suitable manner. Such grouping has been accomplished with the receivers receiving antenna signals of a specific sector only. The grouping, however, has brought on new problems. A problem occurs when combining signals received by the RAKE receivers between receiver groups in different sectors. Combining signals has presented problems, especially in softer hand-off situations, particularly in high-capacity CDMA receivers. There have been attempts to overcome the problems met by routing the signals to a combiner which carries out diversity combining. The routing has, however, produced similar rate problems relating to the bus capacity as mentioned above.
A softer hand-off may be implemented, e.g., by detecting a signal first at a RAKE receiver, after which, instead of diversity combining, substantially the best diversity signal is chosen. This method, however, has not provided an acceptable performance level. An alternative to carrying out a hand-off is optimal combining of RAKE signals and signal detection. However, optimal signal combining is difficult to implement. The outputs of a Walsh-Hadamard transformation typically used at RAKE receivers are connected to the combiner, e.g., via the bus structure. In addition, the implementation is further impeded by the fact that the bus structure must be possible to configure to different kinds of softer hand-off situations dynamically. To implement the solution described above in practice has been difficult and problematic since very high rate requirements are set for the bus. In practice, the combining has been performed by direct summing of the signals of the RAKE receivers. In addition, the signals may have been weighted in different ways in the summing.